feat: use mailbox description for spoolfile

feat: use mailbox description for spoolfile

'spoolfile' is limited to mailboxes paths so if one wants to use a
'named-mailboxes' or 'virtual-mailboxes' as the spoolfile, its path must
be duplicated in the 'spoolfile' assignment. 'virtual-mailboxes' has a
helper variable, 'virtual_spoolfile', for selecting the first
'virtual-mailboxes' entry as the spoolfile.

This commit allows one to use a mailbox description for the 'spoolfile,
which removes the need to duplicate a 'named-mailboxes' or
'virtual-mailboxes' path and makes 'virtual_spoolfile' defunct. The
configuration syntax is now more consistent.

It is important to note that mailbox descriptions are preferred to
paths.

To reflect the change, documentation was updated:
 - Add note to 'spoolfile' about using a string to specify a
   'named-mailboxes' or 'virtual-mailboxes' description.
 - Add a note to the 'virtual_spoolfile' documentation saying that it is no
   longer necessary since 'spoolfile' supports descriptions now.